Reactivity on specific object properties

[cailen-fisher]

Is your feature request related to a problem? Please describe.
Currently, forming a reactive statement around an object property results in the same basic result as reacting to the object at the top level. Making an assignment to any property behaves as if making an assignment to the parent object.

This doesn't seem very intuitive for doing something like $: baz(obj.foo) because then obj.bar += 1 fires baz(obj.foo) even though obj.foo has not changed.

Describe the solution you'd like
The ability to build reactive statements targeting object properties that only react to that property changing via assignment, just like a regular variable.

Describe alternatives you've considered
Currently in this situation I am making an extraneous regular variable, copying the value of the object property I actually want to observe to the regular variable, and then forming the reactive statement around the regular variable.

How important is this feature to you?
Not a deal breaker, but it would be a really nice enhancement

[Conduitry]

Duplicate of #4535.